Composite Disc Golf Mini

[Anthony Fairhurst]

Hi All,

I recently decided to combine two of my passions, composites and disc golf! I started with making a splash mold off a full size disc and it went.. OK. First part tore off the gel coat so luckily I was able to salvage the part and may be able to save the mold. But lacking patience, I decided to try another splash mold and that went worse than the first (see my other post)! I plan to make another splash mold this weekend with some lessons learned in gel coat thickness and patience.

Anyways.. because a carbon disc is dangerous to throw and not legally allowed for PDGA play, I decided to start another project in which I could make and use composite products for tournament play. Mini marker discs ("mini's") are used in disc golf as markers for where your thrown disc lays. The rules state, "Mini Marker discs must have a circular shape, with a diameter ranging from 7 cm to 15 cm and a height not exceeding 3 cm.. mini marker discs can be made from a variety of materials (e.g. plastic, metal, wood)." With that, I modeled a mini mold in CAD and sent it through Xometry.com to get an aluminum mold made. 7 business days later, I received the mold in the mail and spent hours polishing the cavity to a mirror finish.

I originally planned to do wet layup as done with the splash mold but since this is aluminum, I decided to try using prepreg. I laid up the first part in 40 minutes with 4 layers of fabric on the top and chopped bits of prepreg packed into the rim. Since it was 10pm (bedtime), I tightened up the mold and placed it in the oven at 275F for 3hrs. In the morning, I easily removed the part thanks to many layers of release to see a cured part but with a ton of porosity and little to no flash.

2nd part was made with more material and rolled up uni in the bottom rim of the disc. The mold did not close up at first (as expected) so I placed it in the oven at 175F for 1hr and tightened the mold before putting it back. I retightened after another 20 minutes before throwing it back in and turning the oven up to 275F. After getting the part off in the morning I had much more resin flash with less porosity on the faces - less not none!

With that, I am going to try a wet layup so I can get closer to porosity free parts. I also spent a few minutes searching for some expanding sheet material as this is one thing greatly missing from this project - pressure. I was unable to find sheets or films available to the public - only this Syntactic material from Toray.

Does anyone know of such products I can incorporate to get some internal pressure. Or any ideas how to reduce porosity with such a mold?

Some pictures of the tool, process, and parts below;

Pretty Tool - took longer than I'd like to admit to get it to this finish!

 

First layup - cutting up prepreg fabric for a "forged" rim was difficult as it bunched up and turned into unworkable globs. I think next time, I'll try freezing it and cutting it up so it's more flakey. (green mini disc reference in background!)

First part - little to no flash and tons of dry spots

2nd part: Good flash and less porosity

[John Kimball]

Anthony,

porosity in compression molded parts is caused by 1 of 2 things.

1. Not enough material to fill the entire cavity.

2. No place for trapped air to escape thus compressing it into the smallest areas avialable.

For the first one, you should design a tool that has a slightly smaller cavity gap than the Cure Ply Thickness (CPT) of the material. this is easier with prepreg obviously.

For the second, some carefully place vent holes to allow the trapped air and a bit of resin to escape.

I will say that it is a very exacting practice and may take quite a few trial and error attempts to get it right. The best material for cavity compression molding is Sheet Mold Compound (SMC). A basic charge is calculated by weight and then placed in the. As the material is compressed ad heated, it will move to the voids and then excess is expelled through vent holes. This method does not allow for fiber orientation, but does provide excellent surfaces and parts.

The method to get good fiber orientation and excellent surface quality is called SQRTM. This method uses prepreg, liquid resin, and a pressure pot to inject resin into a prepreg filled cavity to fill any voids left by lack of fiber.

Good luck. Can’t wait to see the next iteration. I’d start with adding a bit more material and maybe a few vent channels in the edges.

[Anthony Fairhurst]

Thanks John! the gap between the top and bottom platen is 0.04" so with 4 plys of fabric (CPT 0.011") it should be enough material. On the 2nd iteration I used 6 plys but now believe there simply is no way for air to escape!

What type of vents do you envision? I could Dremel out two small channels from the cavity to resin trap, and resin trap to the pry slots on the edge. Would this suffice?

Along the lines of SQRTM, I was hoping to try a mix of wet and prepreg layup - essentially laying up with prepreg and adding some extra resin into the layup.

Otherwise I'd also like to try a fully wet layup as well as a vacuum cure of each induvial side. However, I will need to devise some special toaster oven. I think there's a good post about that somewhere.. 🙂

[lo_0l]

These look like a great start @Anthony Fairhurst! With a couple of adjustments they should like great 👍

[John Kimball]

15 hours ago, Anthony Fairhurst said:

Thanks John! the gap between the top and bottom platen is 0.04" so with 4 plys of fabric (CPT 0.011") it should be enough material. On the 2nd iteration I used 6 plys but now believe there simply is no way for air to escape!

What type of vents do you envision? I could Dremel out two small channels from the cavity to resin trap, and resin trap to the pry slots on the edge. Would this suffice?

Along the lines of SQRTM, I was hoping to try a mix of wet and prepreg layup - essentially laying up with prepreg and adding some extra resin into the layup.

Otherwise I'd also like to try a fully wet layup as well as a vacuum cure of each induvial side. However, I will need to devise some special toaster oven. I think there's a good post about that somewhere.. 🙂

I’ve been thinking about this today and a few things came to mind. I think you should go with the 4 ply, but maybe consider the orientation of the mold while curing to increase the flow of air to the path of least resistance. maybe up side down or at a 45 degree angle. Also, if your edges are not filled with resin and fiber, then the majority of trapped air will simply fill those voids before it has a chance to leave the mold. 

For vents, try a couple to start at opposite sides of the tool and tilt the tool at a 45 to allow air to travel up. RTM molds will sometimes fill from the bottom and push air out the top as it fills. What you suggest above should work. Start small and adjust from there. I’ve also heard of people vacuum bagging their compression tools to help evacuate air. Yours is small enough to give it a try.

Also,I noticed that you are using a kitchen oven to cure. you have to be careful when doing this as the oven will try to heat up as fast as possible. This can flash cure the surface closest to the heat source and prevent percolation of air in the resin. maybe heat the oven to it’s lowest setting before putting your part in the oven and then slowly increasing the heat by 50F every thirty minutes until you reach cure temp, then hold for 90 - 120 minutes. Ideally, you want to heat the parts at a rate of 2.5 - 5 degrees F per minute. this allows the tool and the material to heat up at the same rate. there’s a lengthy post on how to modify an oven with a temp controller on here somewhere if you’re bored.

[Dan Stout]

Nice work Anthony!  I am incredibly impressed with your mold polishing. Looks like a mirror finish. I can only image how long that took.